INTRODUCTION 5 



siderably even in the same species. It is usually light colored in its early devel- 

 opment and commonly becomes darker as it reaches maturity. The final color 

 may be a light or darker flesh color or a light or darker shade of yellow, orange, 

 red, brown, chestnut, olive, or even black. Whatever the color, it is very seldom 

 the same as that of the thallus. The surface of the disk, further, may be pruinose, 

 concealing the essential color. 



The Exciple. Below the disk is the hymenium, which, in sections, may easily 

 be seen with the hand lens. This structure is usually lighter in color than the disk 

 and is composed of paraphyses and asci. Below the hymenium is the hypothe- 

 cium, often darker in color than the hymenium above it, so that the line of 

 demarcation between the two structures may easily be made out with a hand lens. 

 The hymenium and hypothecium are mentioned here mainly that another structure, 

 the exciple, may be located with reference to them. The exciple is a saucer-shaped 

 or cup-shaped rim around the hymenium, consisting primarily of a continuation 

 of the hypothecium upward on all sides. Such is the proper exciple; but there 

 is sometimes outside of this, or more often replacing it, what is known as a 

 thalloid exciple. This is similar to the thallus in structure, and usually of the same 

 color, which is never true of the proper exciple, the latter usually approaching 

 the disk in color. Either of the exciples may be entirely absent, and either or 

 both may be quite evanescent and seen only in young ascocarps; but usually one 

 of them is present and either permanent or only tardily disappearing. It may be 

 seen readily with the unaided eye or by the aid of the lens, and its nature and 

 degree of development and permanence are points of considerable value in the 

 classification of lichens, even in the determination of species. In the perithecium, 

 the proper exciple has grown completely around the upper part of the hymenium 

 except for a small opening or ostiole at the summit. The exciple thus forms the 

 perithecial wall. The margin of the proper exciple is usually about the level of 

 the outer margin of the disk, or it may be somewhat raised above the disk. This 

 margin is almost always entire, while the margin of a thalloid exciple is frequently 

 crenulate or crenate, or variously branched, ciliate, or irregular. 



Position of the Ascocarp. Sometimes the ascocarp is raised on a slender, 

 upward extension of the thallus, a short stalk or pedicel, quite different in form 

 from the stipe and podetia soon to be described. A stalk is most frequently met 

 in the larger foliose lichens. It may be absent and the ascocarp attached to the 

 thallus at the center of its lower side, in which case; the ascocarp is said to be 

 sessile. Again, the ascocarp may be more closely attached to the thallus by the 

 whole of its lower side, when it is said to be adnate. Finally, the ascocarp may 

 be more or less immersed in the thallus, sometimes deeply, so that when the disk 

 is more or less overgrown by the thallus or by a perithecial wall, the structures 

 are often quite obscured. The development of the ascocarp begins below the 

 surface of the thallus, and the tendency in general is for it to become more and 

 more superficial as maturity is reached. Sometimes, however, it remains perma- 

 nently more or less immersed, and somewhat varying positions with reference to the 

 thallus may be expected in many species. 



Stipes and Podetia. These are structures which serve to raise the ascocarp 

 into the air, and are both to be regarded as primarily important for this purpose. 

 In the genera of Caliciaceae and Cypheliaceae, they have no other function, and 

 are called stipes. In Cladonia and Stereocaulon, however, the stipe takes up, in 

 addition, the structure of the primary thallus with a more or less definite algal 

 layer, and is called a podetium, the proper stipe being devoid of algal cells. The 



